Perimembranous Ventricular Septal Defect—Orientation, Landmarks, and Surgical Implications

Definition, epidemiology, and topography

A perimembranous ventricular septal defect (pmVSD) is a discontinuity of the interventricular septum that involves the membranous septum or its fibrous borders, placing at least one margin of the defect within fibrous tissue contiguous with the central fibrous body [1, 2]. Because this crossroads lies beneath the interleaflet triangle between the right and non-coronary aortic cusps and adjacent to the tricuspid septal leaflet, pmVSDs sit in immediate proximity to both valves and the atrioventricular conduction axis [1, 2, 4]. Perimembranous defects are the most common VSD subtype, accounting for roughly 60–80% of cases across contemporary series [3].

Spatial orientation on a 3D model

• Aortic relationship (anterosuperior rim). The superior/anterior edge abuts the right coronary—and variably the non-coronary—aortic cusp; with outlet extension, it approaches the infundibular (conal) septum [1, 4].
• Tricuspid relationship (right-sided surface). The defect opens to the right ventricle beneath the tricuspid septal leaflet; the trabecula septomarginalis (TSM) and the medial papillary muscle (MPM) often frame the surgeon’s view [1, 4].
• Conduction relationship (inferoposterior rim). After penetrating the central fibrous body, the His bundle runs along the posteroinferior margin of a pmVSD before bifurcating; the right bundle branch (RBB) emerges toward the RV septal surface near the base of the MPM and courses toward the moderator band [1, 4, 8–10].
• Left-sided landmarks. On the LV aspect, the margin tracks the muscular septal crest; the membranous component lies just beneath the interleaflet triangle between the right and non-coronary cusps [1, 2].

Morphologic extensions and why they matter

• Outlet (conal) extension. The anterosuperior rim approaches the infundibular septum and the aortic valve; aortic cusp support may be attenuated, predisposing to cusp prolapse and aortic regurgitation (AR) if unaddressed [1, 4]. The posterior limb of the TSM can partly “roof” the RBB and serves as a visual landmark [4, 10].
• Inlet extension. Encroachment toward the AV-valvar septum brings the RBB closer to the visible RV margin, increasing the conduction risk along the inferior rim [2, 4].
• Trabecular (mid-muscular) extension. Adds muscular borders without altering the defining fibrous component; conduction proximity remains greatest posteroinferiorly [1, 4].

Natural history, hemodynamics, and presentation

Left-to-right shunting is driven by LV–RV systolic gradients. Small, restrictive defects generate minimal shunt and soft systolic murmurs, whereas moderate–large shunts cause LV volume loading, pulmonary over-circulation, and heart-failure symptoms in infancy. pmVSDs are less likely to close spontaneously than muscular defects; neonatal cohort data show ~51% spontaneous closure for pmVSD versus ~97% for muscular VSD over seven years, with defects ≥4 mm less likely to close [15]. Outlet extension increases the risk of right coronary cusp prolapse and progressive AR, converting an isolated VSD into a combined lesion [3, 4].

Imaging strategy and 3D “surgical views”

• Transthoracic echocardiography (TTE) delineates membranous involvement, AV/TV relationships, and cusp prolapse; targeted short-axis and inflow–outflow views define anterosuperior and inferior rims. Real-world protocols in 100 consecutive patients demonstrate that 3D echo reliably provides en-face views that match operative findings and standardize communication with surgeons [6].
• 3D echocardiography adds unconventional planes unobtainable by 2D imaging and improves assessment of rims, valve–defect relationships, and suitability for device or surgical closure [7].
• Transesophageal echo (TEE, intraoperative) confirms patch seating, tricuspid competence, and freedom of aortic cusp motion.
• CT/MRI is reserved for complex morphologies, associated lesions, or pre-procedural planning when echo windows are limited [1].

Surgical anatomy translated to safe technique

Principle: Respect the inferoposterior (conduction) rim; manage the anterosuperior (aortic) rim to prevent leaflet distortion.

1. Patch introduction (“parachute”) at the anterosuperior margin.

This zone is typically free of conduction tissue, permitting early orientation with minimal block risk; avoid bites that tether the interleaflet triangle or aortic annulus [1, 4, 11].

2. Running the anterosuperior suture line.

Track just inside the LV side of the rim to preserve cusp support; keep a shallow needle trajectory near the aortic annulus. With outlet extension, be mindful of the infundibular septum and right coronary cusp; modest oversizing can prevent annular traction [1, 4, 11].

3. Posteroinferior (conduction) rim—high-alert zone.

The His–RBB axis hugs this border. Favor shallow bites slightly away from the true edge on the RV side of the septal crest, minimizing penetration toward the conduction plane [8–10, 13]. Contemporary series suggest that shallower, edge-hugging sutures may reduce postoperative RBBB and tricuspid regurgitation compared with traditional deeper bites placed several millimeters from the rim [9, 13].

4. Tricuspid valve stewardship.

Avoid leaflet plication or mal-height from aggressive bites across the septal leaflet. After the inferior arc, confirm coaptation height and chordal freedom; correct subtle leaflet tenting before tying [1, 11].

5. Knot management and exit.

Externalize and tie on the right atrial side to reduce intraventricular bulk; reinspect aortic cusps and tricuspid valve; use saline testing and TEE to detect occult issues [1, 11].

Outcomes and pitfalls to anticipate

Elective pmVSD repair in experienced centers carries very low mortality and excellent freedom from reintervention when valve stewardship and conduction-conscious suturing are applied [11, 12]. The principal preventable complications include:

• Heart block. Deep or medial bites along the posteroinferior rim can injure the His–RBB axis; late-presenting complete heart block, although uncommon, is well documented and underscores the need for lifelong follow-up [12, 13].
• Aortic leaflet impingement / new AR. High bites near the interleaflet triangle or patch tension on the annulus are typical culprits [1, 3, 4].
• Tricuspid regurgitation. Leaflet distortion from non-anatomic bites or excessive traction can precipitate TR; shallow edge suturing mitigates this [9, 13].
• Residual shunt. Most often at commissural turns or intentionally shallow segments; meticulous inspection with TEE enables targeted reinforcement [11, 12].

Decision points beyond anatomy

Timing is guided by shunt magnitude, symptoms, pulmonary over-circulation, and onset of AR [1, 3]. Approach is tailored to exposure and valve relationships; pmVSDs are frequently amenable to a right-atrial (tricuspid) approach with excellent visualization of critical rims [1, 4, 11]. Associated lesions (double-chambered RV, subaortic ridge, cleft TV, etc.) should be corrected concomitantly to optimize durability [1, 4]. In select anatomies, electro-anatomic mapping during percutaneous procedures reinforces the posteroinferior course of the conduction system and may refine risk stratification for device closure; however, device-related AV block remains a recognized hazard and informs case selection [8, 14].

Key take-home points

• pmVSDs are the predominant VSD subtype and lie at the intersection of the aortic valve, tricuspid valve, and AV conduction axis [1–4].
• 3D echocardiography provides reproducible en-face “surgeon’s views” that directly support procedural planning [6, 7].
• Conduction-conscious, shallow edge suturing and meticulous valve stewardship minimize heart block, TR, and AR [8–13].
• Lifelong surveillance is prudent given the small but real risk of late AV block and the possibility of progressive aortic cusp changes in outlet-related morphologies [3, 13].

References

[1] Spicer DE, Hsu HH, Co-Vu J, Anderson RH, Backer CL. Ventricular septal defect. Orphanet J Rare Dis. 2014;9:144.

[2] Tretter JT, Mori S, Smerup M, et al. Assessing the criteria for definition of perimembranous ventricular septal defects in light of the search for consensus. Orphanet J Rare Dis. 2019;14:76.

[3] Dakkak W, Oliver TI. Ventricular Septal Defect. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024.

[4] Anderson RH, Wilcox BR. The surgical anatomy of ventricular septal defect. J Card Surg. 1992;7(1):17-35.

[5] Mostefa-Kara M, Ou P, Bonnet D, et al. Anatomy of the ventricular septal defect in congenital heart disease. Cardiovasc Diagn Ther. 2018;8(6):753-765.

[6] Cossor W, Cui VW, Roberson DA. Three-Dimensional Echocardiographic En Face Views of Ventricular Septal Defects: Feasibility, Accuracy, Imaging Protocols and Reference Image Collection. J Am Soc Echocardiogr. 2015;28(9):1020-1029.

[7] Mejia AAS, Manrique RA, Castillo GH, et al. Utility of 3-Dimensional Echocardiography in Patients With Ventricular Septal Defects. Cureus. 2024;16(7):e65829.

[8] Kaur D, Koneti NR, Jajoo S, et al. Mapping the Conduction System in Patients Undergoing Transcatheter Device Closure of Perimembranous Ventricular Septal Defect: A Proof-of-Concept Study. Pediatr Cardiol. 2022;43(3):674-684.

[9] Varghese R, Saheed S, Ravi AK, Sherrif EA, Agarwal R, Kothandam S. The “excluding” suture technique for surgical closure of ventricular septal defects: a retrospective study comparing the standard technique. Ann Pediatr Cardiol. 2016;9(3):229-235.

[10] Tamiya T, Kurosawa H, Becker AE. A histological study of surgical landmarks for the atrioventricular conduction system in ventricular septal defect. Thorac Cardiovasc Surg. 1985;33(6):344-348.

[11] Scully BB, Morales DL, Zafar F, et al. Current Expectations for Surgical Repair of Isolated Ventricular Septal Defects. Ann Thorac Surg. 2010;89(2):544-549; discussion 550-551.

[12] Ergün S, Genç SB, Yildiz O, et al. Risk Factors for Major Adverse Events after Surgical Closure of Ventricular Septal Defect in Patients Less than 1 Year of Age: A Single-Center Retrospective Study. Braz J Cardiovasc Surg. 2019;34(3):335-343.

[13] Altaweel H, Kabbani MS, Hijazi O, Hammadah HM, Al Ghamdi S. Late presenting complete heart block after surgical repair of ventricular septal defect. J Saudi Heart Assoc. 2018;30(4):301-305.

[14] Predescu D, Chaturvedi RR, Friedberg MK, et al. Complete heart block associated with device closure of perimembranous ventricular septal defect. Catheter Cardiovasc Interv. 2008;71(6):791-797.

[15] Zhao QM, Niu C, Liu F, et al. Spontaneous Closure Rates of Ventricular Septal Defects (6,750 Consecutive Neonates). Am J Cardiol. 2019;124(4):613-617.

(Heart model used with permission from CrossMedical, Inc.)